Instruments and implants such as surgical instruments and orthopedic implants are manufactured according to specifications usually illustrated in manufacturing drawings. The manufacturing drawings specify dimensions and precision requirements for the manufactured instruments. These precision requirements are stricter when the instruments or implants are used in an environment such as is encountered in a Computer Assisted Surgery (CAS) system.
After the manufacturing of an instrument, comparative measurements of the manufactured instrument are made with the initial specifications. If the end result of the measurements is outside the specifications of the manufacturing drawings, the instrument is rejected. To achieve high precision, the manufacturing process can be expensive.
A CAS system creates a precision environment where a surgeon uses a computer system to track, in a 3-dimensional reference spatial system, one or more instruments and implants. The precision required varies from 0.1 mm to 1 mm in position and can also be very high in angle. The instruments and implants tracked by the CAS system have generic characteristics that need to be known by the CAS system. In addition, the CAS system needs to track the relative position of the implant or the instrument to the tracker coordinate system. This is typically done using one of various calibration techniques.
A first known calibration method consists in identifying the tip and the axis of a tool with the help of a calibration block. The block has a base plate with a pin hole located at its center to position the tip of the instrument. Around the pin hole, eight posts are placed in a quasi-circular position. The tool is equipped with a means for registering and tracking the tool in a 3D environment. For registering the tip of the instrument, the tip is positioned against the pinhole located at the center of the base plate. The system registers both the calibration block and the instrument and calculates the position of the tip of the instrument from its position in the pinhole of the calibration block. To determine the axis, the instrument is successively positioned against the eight posts located on the calibration block and registered. A second calibration method consists in using a simplified calibration block capable of positioning the tool against a reference pinhole and clamp in a known position. The system registers both the calibration block and the instrument. From the registration of the tool, the system can extrapolate the position of the tip and, since the calibration block has clamped the instrument in a known position, the system can extract the axis of the tool from the registration of the tool and the calibration block and the known position of the clamped instrument.
There is a need for a method to calibrate a tool or implant that would reduce the time spent in the operating room performing the calibration, and simplify the procedure.